Eolian Environments - Trace Fossils as Indicators of Sedimentary Environments - page 413

3.2 Trace Fossils in Inland Ergs
Inland ergs (vast expanses of high, largely unvegetated, sand dunes) occur
in interior locales on several continents today. Some of the most extensive
modern ergs include the well-known Sahara of northern Africa, Namib of south-
western Africa, Negev in Israel, and Atacama in Chile and Peru. The world's
largest erg today is the Rub'al Khali (Empty Quarter), which covers more than
650,000 km 2 in the southern Arabian Peninsula with vast expanses of sand
dunes exceeding 250 m in height. Smaller ergs in western North America,
which have been studied extensively by sedimentologists and desert ecologists,
include Great Sand Dunes in Colorado, Little Sahara and Coral Pink Sand
Dunes in Utah, White Sands in New Mexico, southern Mojave Desert in
California, and Gran Desierto de Altar in Sonora, Mexico. Similarly, western
North America was a region of widespread ergs off and on throughout the
Permian, Triassic, and Jurassic periods ( Kocurek and Dott, 1983; Peterson,
1988 ). These ancient eolianites have been studied extensively by sedimentolo-
gists, many of whom have noted common tetrapod tracks and other trace fossils
in those deposits ( Fig. 3 ).
Vertebrate footprints have been widely reported in eolianites, and dinosaur
tracks and trackways in dune deposits are especially well documented in the
scientific literature and widely publicized in the popular media. Images of dino-
saurs and evidence of dinosaur lifestyles fascinate paleontologists and nonsci-
entists alike, so it is no wonder that dinosaur footprints are among the most
frequently reported trace fossils in eolian deposits.
Jurassic ergs in the southwestern Unites States, particularly in southern Utah
and northern Arizona, were favored habitats of dinosaurs and other primitive
tetrapods. Their bones are exceedingly rare, but their tracks are amazingly
abundant ( Hamblin and Foster, 2000; Lockley, 1998; Lockley and Hunt, 1995;
Lockley et al., 2007; Loope, 2006b; Mil`n and Loope, 2007; Rainforth and
Lockley, 1996; Seiler and Chan, 2008; Thulborn, 1990 ). Well-preserved foot-
prints provide direct evidence of the various kinds of desert-dwelling dinosaurs
in the Jurassic, as well as information about their size, foot morphology, and
even skin texture. Their trackways offer information about the dinosaurs' gait
and perhaps even speed as they traversed the vast Jurassic dune fields. The trace
fossil Grallator , a three-toed track generally attributed to theropod dinosaurs, is
one of the most common ichnogenera in the eolianites ( Fig. 4 ). As theropods
were voracious predators, their apparent abundance in the Jurassic ergs demon-
strates that there must have been a substantial number of prey animals inhabit-
ing those environments also.
Preservation of tetrapod tracks in eolian deposits includes both surface impres-
sions and undertracks. As the bulk of preserved eolian deposits represent the cross-
bedded slip faces of dunes, vertebrate tracks typically exhibit some deformation
features. Loope (2006b) described the details of preservation modes of abundant
tetrapod (dinosaur and therapsid) tracks in grain-flow layers of slip face deposits